Passive speed and power regulation of a wind turbine

ABSTRACT

A wind turbine with at least one blade having a pitch axis that is offset laterally so that the blade does not intersect with an axis of rotation of the wind turbine, and a center of mass and an aerodynamic center are offset from the pitch axis in the direction parallel to the axis of rotation of the wind turbine. The blade is arranged so that the blade can pitch towards stall against the action of a resilient device and also so that the centrifugal loading on at least one of the blade and one or more components optionally attached to the blade, acts against the spring in a direction to shed power by pitching the blade towards stall and also so that the torque loading provided by a power conversion device acts to pitch the blade in the opposite direction, so as to reduce the amount of load shedding whenever the power is being extracted from the wind turbine.

This invention relates to a passive speed and power regulation systemfor a wind turbine.

Wind turbines are well known devices used to extract energy from thewind. Most commonly these are horizontal axis machines with two or moreblades. A key design challenge for all such machines is the need toregulate the power extracted from the wind to avoid excessive rotorspeed or overload of the generator in high winds. One method ofachieving this is through altering the pitch angle of the blades. Thiscan be achieved either through active regulation (for example usingelectrical or hydraulic actuators) or passive means. Passive regulationinvolves the use of the forces that are naturally present within thewind turbine to pitch the blades against a spring or deform the bladesor their attachment points.

The forces that are naturally present include the centrifugal loadsacting on the wind turbine blades, the torque in the main shaft and thethrust load acting axially along the main shaft. These forces may beused in many different physical configurations to pitch the blades.Pitching of the blades may either be towards feather (to reduce theaerodynamic angle of attack and therefore reduce the aerodynamic liftforce that is providing the power to the wind turbine) or towards stall.Pitching towards stall increases the angle of attack and can thereforeincrease power initially, but eventually the pitch angle will be reachedwhere the blades will ‘stall’ and the aerodynamic lift will be lost andthe aerodynamic drag will increase. Thus the wind turbine will lose itsability to produce power.

There are several existing methods used to achieve passive blade pitch,including methods that use each of these forces. This patent describes adifferent method, which exploits the combination of both bentrifugalloads on the blades, and shaft torque loads to give an improved methodof pitch control that works irrespective of whether the generator isproviding electrical power. It is not only the combination of theseforces that is significant, but also the direction in which theyoperate. The centrifugal load acts to shed power by pitching the bladestowards stall and the shaft torque load acts in the opposite directionto reduce the amount of blade pitching and therefore reduce the amountof power-shedding.

The present invention involves the combination of centrifugal loading onthe wind turbine blades, or components attached to them and the use ofthe generator reaction torque, to provide protection of a wind turbineagainst over-speeding and against producing too much power. The methodof protection works irrespective of how much power the consumer isdemanding of the wind turbine through the generator. The protection isachieved through passive pitch regulation, such that the centrifugalloading acts to try to pitch the blades away from the optimum positionfor energy capture towards stall so as to shed power. The generatorreaction torque (if present) acts in the opposite direction to reducethe amount of pitch movement and therefore reduce the amount of powershedding.

A geometric configuration is presented that achieves this combination ofattributes.

According to the invention, there is provided a wind turbine as definedin claim 1. More specific and/or preferred features of the invention areset out in the dependent claims.

In a specific embodiment of the invention a means of attaching theblades to the wind turbine results in a partial cancellation of some ofthe local loads normally imposed on the hub. This is achieved byattaching each blade both at its end and at a second outer location. Theblades are arranged so that each outer blade attachment point is asclose as possible to the adjacent blade's end attachment point. Sincethe loads associated with adjacent blades are largely equal andopposite, this greatly reduces the severity of the local loadstransferred to the hub from the blades. This is particularly beneficialif the blades are attached directly onto the rotor of a direct drivegenerator, since it is important to minimise distortion of the generatorand the variation in the generator air gap.

It is desirable that the turbine blades are linked so that they haveidentical pitch setting at all times. Two different means of achievingthis are also described.

The first method involves a compact means of linking a number of bladeswith the geometry outlined above, applying a preload and resilientresistance to blade pitch, and allowing the blade to be linked in pitchwhilst minimising the loads carried by the linkage mechanism.

This method uses pushrods attached to a pitch lever on each blade whichact against a resilient member such as a compression spring. The pushrodis restrained in its movement in order to act as a fixed stop to definethe optimum blade pitch position and in conjunction with the spring toboth apply a preload and offer a resistance in proportion to furtherblade pitch.

A profiled pitch linkage plate is mounted such that it is able to rotateabout the main axis of the turbine. The apices of this plate are eachconnected to one of the blade pushrods in such a manner that the platewill rotate as a consequence of blade pitch thereby constraining theblades to ensure identical pitch. By connecting the plate directly tothe pushrods the loads transmitted by the rotating plate are reduced byeliminating the need to transmit pitch preload or pitch resistanceloads.

The second method has the additional benefits of offering both a meansof stopping the turbine completely in high winds, and providing adamping force proportional to the rate of change of blade pitch. Thiscan improve the dynamic characteristics of the pitch motion.

In this arrangement rigid members known as pitch arms are attached tothe blades so that they rotate when the blades pitch. The other ends ofthese pitch arms are connected by means of a pinned joint to a rigidannulus which is allowed to rotate about the rotor axis. The arcdescribed by the end of each pitch arm due to rotation of the blade inpitch has two components, one parallel to the main axis of the turbine(the ‘axial component’) and one in the plane of rotation of the hub(‘the in-plane component’).

The in-plane component of the arc described by the end of each pitch armresults in rotation of the annulus thereby restraining each blade toidentical pitch angle and change of pitch.

The axial component of the arc described by the end of each pitch armdisplaces the annulus axially. If the annulus is restrained axially bymeans of a resilient member which provides a resistance proportional tothe degree of rotation of the blade in pitch, and the axial movement ofthe annulus is limited in one direction by means of fixed stops in orderto give a set initial blade pitch angle, a pre-load can be applied tothe resilient member so that the blade pitch moment must reach a setlevel before the blade begins to change pitch.

The resilient biasing member may comprise a circular diaphragm which canbe elastically deformed in an out of plane direction such that axialmovement away from its equilibrium position results in a resistanceproportional to its deflection. The circular diaphragm is constrainedaxially and radially but is allowed to rotate about the rotor axis. Apreload may be introduced in the diaphragm spring by restraining theblades from pitching past a certain point using fixed stops and bysecuring the centre portion of the diaphragm at a position axiallyremoved from its un-deformed position.

The rigid annulus may be connected to a surface so disposed withrelation to the wind direction that due to its significant aerodynamicresistance to the wind it exerts an axial force on the annulus whichincreases with the wind speed.

The force exerted by this surface acts to move the rigid annulus andhence the ends of the pitch arms axially thereby causing the blades topitch towards a stall position once the pre-load in the diaphragm isovercome by the combination of blade pitch moment due to centrifugalforces and the aerodynamic force on this surface. This will tend tomaintain the blades in stall in extreme wind conditions and result inlower rotational speeds. The axial movement of the surface which resultsfrom a change in pitch of the blades due to rotation about their pitchaxes is also resisted by a force proportional to the speed of axialdeflection due to the air resistance of the surface thereby providing adamping force to oppose changes in blade pitch.

This damping force will increase in proportion to the rate of change ofblade pitch thereby resisting rapid changes in blade pitch.

An example of a geometric layout that achieves the requirements forblade passive pitch presented above, and examples of how this layout maybe realised will now be described with reference to the followingdrawings in which:—

FIG. 1 shows the geometry of the blade attachment and pitch axis;

FIG. 2 is a view on the main axis of the turbine rotor of a firstembodiment of the invention showing the arrangement of the blades on atriangular hub plate with the pitch control components removed forclarity;

FIG. 3 is a side elevation of the turbine rotor showing the arrangementof one blade attached to the hub and with the pitch control componentsand other blades removed for clarity;

FIG. 4 is a view on the underside of one blade showing the pitch lever,pushrod, compression spring and spring fixture;

FIG. 5 shows a modification of the first embodiment of the invention inwhich the blade and pushrod assembly are mounted on a circular hub plateand connected to the pitch linkage plate, with blades not shown forclarity;

FIG. 6 shows the blade and pushrod assembly mounted on a circular hubplate and connected to the pitch linkage plate with blades shown;

FIG. 7 is a view on the upwind root area of the blade of a secondembodiment of the invention, showing the attachment of the outer bladepitch bearing and pitch arm and the blade end pitch pin;

FIG. 8 is an isometric view of the turbine rotor from a downwinddirection showing the inter-connection of the blades' pitch arms by therigid annulus;

FIG. 9 is an isometric view of the turbine rotor from an upwinddirection showing the rigid annulus located on the main shaft by thediaphragm spring member; and

FIG. 10 is a side elevation of the turbine rotor showing the threeblades connected to the rigid annulus and diaphragm spring damper.

A geometric arrangement that achieves the requirements for blade passivepitch is shown in FIG. 1. The figure shows only one blade for clarity.The turbine main axis is shown by (Z-Z), the direction of rotation by(R), and the direction of the wind by (U). The blade is mounted so thatit can rotate about a pitch axis (X-X), which is offset radially by adistance (y) relative to the axis of the main shaft (Z-Z). The bladeaxis (B-B) is inclined at an angle (A1) to the pitch axis (X-X). Thecentre of mass of the blade (m) lies on the blade axis (B-B).

A pre-loaded resilient member such as a spring (D), holds the bladeagainst a hard stop at the optimum pitch position for maximising energycapture. The wind turbine blade is arranged so that it can pitch towardsstall, but is restrained from doing so by the preloaded spring. Theblades are held at this optimum pitch angle until the loads acting topitch the blades are sufficient to overcome the pre-load and start topitch the blades towards stall in the direction of arrow (P).

This arrangement has two characteristics. Firstly, the centrifugalloading acting on the blade when the wind turbine is rotating (Fc) has acomponent that acts to try to pitch the blade about the pitch axis(X-X). It is arranged so that this force acts to try to overcome thepreloaded spring and pitch the blade towards stall so as to shedaerodynamic power.

Secondly, if the blade is producing power, then it will be generating anaerodynamic torque about the axis of the main shaft (Z-Z), as indicatedby arrow (R). If the generator is not generating power it will not beproviding a reaction to this torque and the aerodynamic torque willsimply cause the rotor to accelerate and it will not directly influencethe pitch angle (provided that most of the rotational inertia of thesystem is in the blades and not the generator). However, if thegenerator is connected to a load, then it will provide a reactive torquethat may or may not be sufficient to prevent the rotor fromaccelerating. This reactive torque also has a component that acts to tryto pitch the blades. The geometry is such that the generator torque actsto pitch the blade away from stall towards the optimum energy captureposition.

The key to understanding the benefits of this geometry require acomparison between what happens when the generator is not connected to aload (and the wind turbine is freewheeling) and when it is connected toa load and is producing power.

If the generator is not connected to an electrical load, then the windturbine will effectively be freewheeling. At the maximum allowablespeed, the spring force can be set so that the centrifugal force is ableto overcome the pre-load and pitch the blades sufficiently to preventany aerodynamic power generation, so as to prevent any furtheracceleration (typically this will be 15 to 20° towards stall).

If the wind turbine generator is producing power in light to moderatewind, no pitching will be required since the generator will be able tolimit the speed of the wind turbine. In high winds some pitching will berequired to ensure that at the same maximum allowable rotational speed,the aerodynamic power produced does not exceed that being converted toelectricity by the generator, so as to prevent any further acceleration.This will require a smaller change in pitch than required if thegenerator were not in use. The geometry of the wind turbine can beconfigured to ensure that the generator reaction torque makes sufficientcontribution to the pitch moment to reduce the amount of load sheddingby just the required amount.

Therefore, this combination of features allows the wind turbine to havea defined maximum allowable rotational speed that will not be exceededirrespective of whether the generator is connected to a load or the windturbine is simply freewheeling.

Additionally, geometry suited to the requirements for the passive pitcharrangement detailed above can be arranged so that the severity of theloads transferred to the hub is greatly reduced. This is illustrated inFIGS. 2 and 3 in which three blades (1), which rotate about a shaft (4),are each attached to the hub plate (2) both at their ends (11) and at asecond outer location (12). The blades (1) are arranged so that eachouter blade attachment point (12) is as close as possible to theadjacent blade's end attachment point (11). Since the loads associatedwith adjacent blades are largely equal and opposite, this greatlyreduces the severity of the local loads transferred to the hub from theblades.

It is preferable that the turbines blades have identical pitch settingat all times. In order to achieve this it is necessary for the blades tobe mechanically linked. Two different methods of ensuring this matchingof blade pitch setting are proposed.

In the first method, (see FIG. 4), each blade (1) is rotatably mountedon a blade end attachment (11) and an outer blade attachment (12). Apitch lever (13) is rigidly attached to the underside of the blade sothat it rotates with the blade (1) in pitch. The pitch lever (13) isconnected to a pushrod (14) which is acted on by a spring (15) and alsois constrained by a spring bracket (16). This both acts as a fixed stopso that the blade can only pitch in the direction towards stall andapplies a preload to the spring (15). The spring bracket (16) andpushrod (14) hold the blade (1) at a set position against the preload ofthe spring (15).

The means of linking the three blades in pitch is illustrated in FIG. 5and FIG. 6 which show an arrangement with three blades mounted on acircular hub plate. In order to link the blades in pitch a profiledplate (17) with connection points at each of its three apices isrotatably mounted on the turbine main axis parallel to the hub plate.Each pushrod (14) is attached to one of the connection points byradially compliant joint (18). Any change in blade pitch causes rotationof the plate (17) and an identical change in the pitch of all blades.

By separating the functions of the rotating pitch linkage plate and therequirement for a preload and resilient resistance to pitch, the loadstransmitted by the pitch linkage plate are considerably reduced.

In the second method, the blades are not only connected in pitch, butare also subjected to a pitching moment towards stall that increaseswith increasing wind speed. This feature is able to bring the windturbine to rest in extreme wind speeds.

FIG. 7 shows the location of the blade outer attachment (8) & (9) andpitch arm (5) which are rigidly attached to the upwind surface of theblade (1). It also shows the blade end pin (10) which is co-axial withthe pitch axis and the outer blade attachment pin (9).

A means of inter-connecting the blades is shown in FIG. 8. The threeblades (1), (drawn with dotted lines for clarity), are connected viatheir respective pitch arms (5) to a rigid annulus (6). Rotation of theblades (1) about their pitch axes causes both rotation and axialdeflection of the rigid annulus (6) and thus each blade (1) isrestrained to rotate about its pitch axis by an equal amount.

FIG. 9 shows the turbine rotor from the upwind direction with the blades(1) connected to the rigid annulus (6) by pitch arms (5). The rigidannulus (6) is attached to a diaphragm (7) which is located by itscentre portion on the shaft (4), such that it is restrained in theradial and axial directions but free to rotate around the main axis ofthe rotor.

As the diaphragm (7) is restrained axially at its centre it is forced todeflect as the outer annular rim (6) moves axially and thereby behavesin such a way as to offer a biasing force to oppose this movement. Thediaphragm (7) is positioned axially such that it acts as a biasing forceto rotate the blades (1) about their pitch axes until they contact afixed stop positioned to maintain them at a pre-selected pitch angle.

FIG. 10 shows a side elevation of the turbine rotor with the diaphragm(7) deflected to give a pre-loaded spring bias to hold the blades (1)against the fixed stops until the pitch moment is sufficient to causechange in blade pitch. Therefore, the diaphragm is able to replace thefunction of the pre-loaded spring referred to in the previous case. Italso provides a degree of pitch linkage that can replace the pitchlinkage system described previously.

The pitch control diaphragm (7) is also arranged such that it will beacted upon by the full force of the prevailing wind. Careful sizing ofthis surface allows a predictable additional axial force to be appliedto the annular outer rim (6) which is connected to the pitch arms whichcontrol blade pitch. Thus in a strong wind the aerodynamic resistance ofthis surface will automatically move the blades to a stall position.This system can be used to bring the wind turbine to rest in high winds.

The axial movement of the surface in conjunction with the change inpitch of the blade allows the surface to contribute a damping forceproportional to the rate of change of pitch angle. This damping force isderived from changes in the wind speed acting on the diaphragm due toits movement and this can contribute significantly to improving thedynamic behaviour of the pitch system.

1. A wind turbine with at least one blade having a pitch axis that isoffset laterally so that the blade does not intersect with an axis ofrotation of a rotor shaft of the wind turbine, and a center of mass andan aerodynamic center are offset from the pitch axis in the directionparallel to the axis of rotation of a rotor shaft of the wind turbine,the blade being arranged so that the blade can pitch towards stallagainst the action of a resilient device, and also so that thecentrifugal loading on at least one of the blade, and one or morecomponents attached to the blade, acts against the spring in a directionto shed power by pitching the blade towards stall and also so that thetorque loading provided by a power conversion device acts to pitch theblade in the opposite direction, so as to reduce the amount of loadshedding whenever the power is being extracted from the wind turbine. 2.A wind turbine as claimed in claim 1, wherein the wind turbine comprisestwo or more blades, wherein a pitch angle of each of the blades is madethe same through a common pitch linkage system.
 3. A wind turbine asclaimed in claim 2, wherein the common pitch linkage system comprises aplate that rotates about the axis of rotation of the rotor shaft of thewind turbine.
 4. A wind turbine as claimed in claim 1, wherein the windturbine comprises two or more blades, wherein each of the blades isattached at a blade end connection point and at an outboard connectionpoint and wherein the blades are arranged so that the blade endconnection of each of the blades is adjacent to or coincident with theoutboard connection of for another one of the blades.
 5. A wind turbineas claimed in claim 4, wherein the blades are attached directly onto therotor shaft.
 6. A wind turbine as claimed in claim 1, wherein the windturbine comprises two or more blades wherein the blades are linked toeach other by members, each of the pitch members is connected at one endto one of the blades such that they rotate with changes in pitch of theblade and to a rigid annulus at the other end which is constrainedradially, but is allowed to rotate about the axis of rotation of therotor shaft of the wind turbine.
 7. A wind turbine as claimed in claim6, wherein the annulus linking the pitch members is constrained axiallyby a resilient member which provides a biasing force such that axialmovement away from an equilibrium position of the annulus results in aresistance due to deflection of the resilient member which acts on thethe pitch members connected to the annulus in order to oppose rotationof the blades about their pitch axis.
 8. A wind turbine as claimed inclaim 7, further comprising fixed stops which set the initial pitchangle of the blades and apply a pre-load to the resilient member whichmaintain the blades at a nominal pitch angle until a selected pitchmoment is achieved.
 9. A wind turbine as claimed in claim 7, wherein theresilient member is a circular diaphragm.
 10. A wind turbine as claimedin claim 9, wherein the circular diaphragm is subjected to a pre-load byrestraining the blades from pitching past a certain point using fixedstops and by securing a center portion of the diaphragm axially removedfrom the equilibrium position of the diaphragm.
 11. A wind turbine asclaimed in claim 7, wherein the annulus is connected to a surface sodisposed with relation to wind direction that an aerodynamic resistanceof the surface exerts an axial force on the annulus which varies withthe wind speed.
 12. A wind turbine as claimed in claim 11, wherein theaxial force exerted by the surface acts to move the ends of the pitchmembers axially thereby causing the blades to pitch.
 13. A wind turbineas claimed in claim 11, wherein the axial movement of the surface whichresults from a change in pitch of the blades due to rotation about theirpitch axes is resisted by a force proportional to the speed of axialdeflection due to the air resistance of the surface thereby providing adamping force to oppose rapid changes in the pitch of the blades.